Reaction Kinetics

21 free MDCAT Chemistry MCQs on Reaction Kinetics (PM&DC syllabus), each with the answer, an explanation and the textbook line it comes from. Tap to check, no sign-up.

Tap an answer to check it · 21 questions

1. Question 1

   In Fig. 9.1 a fall of 0.1 mol/dm3 in reactant concentration takes 15 minutes at the start. At a later stage the same fall takes:
   • A15 minutes
   • B150 minutes
   • C1.5 minutes
   • D30 minutes
   Show answer and explanation

   Answer: B) 150 minutes

   Rate falls as reactant concentration falls, so the same change takes longer later. Source: Chemistry 11 KPK (taleem360.com), p. 277.

2. Question 2

   The branch of chemistry dealing with the rates of reactions, the factors affecting them and their mechanisms is:
   • AThermochemistry
   • BChemical kinetics
   • CElectrochemistry
   • DChemical equilibrium
   Show answer and explanation

   Answer: B) Chemical kinetics

   Kinetics asks how fast; thermochemistry asks how much heat. Source: Chemistry 11 KPK (taleem360.com), p. 276.

3. Question 3

   The change in concentration of a reactant or product per unit time is the:
   • AOrder of reaction
   • BRate constant
   • CRate of reaction
   • DActivation energy
   Show answer and explanation

   Answer: C) Rate of reaction

   Units: mol dm-3 s-1. Source: Chemistry 11 KPK (taleem360.com), p. 276.

4. Question 4

   In a multistep mechanism, the slowest step that controls the overall rate is the:
   • AReaction intermediate
   • BElementary step
   • CActivated complex
   • DRate determining step
   Show answer and explanation

   Answer: D) Rate determining step

   The overall rate can be no faster than its slowest step. Source: Chemistry 11 KPK (taleem360.com), p. 278.

5. Question 5

   The sum of the exponents of the concentration terms in the rate expression is the:
   • AOrder of reaction
   • BMolecularity
   • CRate constant
   • DRate law
   Show answer and explanation

   Answer: A) Order of reaction

   e.g. rate = k[A]^2[B] is third order. Source: Chemistry 11 KPK (taleem360.com), p. 280.

6. Question 6

   The activated complex is a substance which is:
   • AUnstable
   • BStable
   • CEasily isolated
   • DA final product
   Show answer and explanation

   Answer: A) Unstable

   It has partial bonds, a short life, and can go on to products or back to reactants. Source: Chemistry 11 KPK (taleem360.com), p. 290.

7. Question 7

   For A -> products, doubling the concentration of A quadruples the rate. The reaction is:
   • AFirst order
   • BSecond order
   • CZero order
   • DThird order
   Show answer and explanation

   Answer: B) Second order

   Rate = k[A]^n with 2^n = 4, so n = 2. Source: Chemistry 11 KPK (taleem360.com), p. 282.

8. Question 8

   An increase in temperature increases the rate of reaction mainly because:
   • AFewer collisions take place
   • BActivation energy is lowered greatly
   • CMore particles reach activation energy
   • DMolecules become much heavier
   Show answer and explanation

   Answer: C) More particles reach activation energy

   A larger fraction of molecules has energy equal to or greater than Ea (shaded area in Fig. 9.3). Source: Chemistry 11 KPK (taleem360.com), p. 288.

9. Question 9

   Which condition will increase the rate of a reaction?
   • AHigher temperature, lower concentration
   • BLower temperature, lower concentration
   • CLower temperature, higher concentration
   • DHigher temperature, higher concentration
   Show answer and explanation

   Answer: D) Higher temperature, higher concentration

   Both raising temperature and raising concentration speed up a reaction. Source: Chemistry 11 KPK (taleem360.com), p. 287.

10. Question 10

    The rate of a reaction generally decreases with time because:
    • AReactant concentration decreases
    • BTemperature always falls
    • CCatalyst is used up
    • DActivation energy rises
    Show answer and explanation

    Answer: A) Reactant concentration decreases

    Fewer reactant particles mean fewer collisions per unit time. Source: Chemistry 11 KPK (taleem360.com), p. 278.

11. Question 11

    The rate determining step of a mechanism is also the step with the:
    • ALowest activation energy
    • BHighest activation energy
    • CMost intermediates
    • DFastest rate
    Show answer and explanation

    Answer: B) Highest activation energy

    The highest barrier makes it the slowest step. Source: Chemistry 11 KPK (taleem360.com), p. 278.

12. Question 12

    Reaction intermediates in a mechanism are:
    • AThe final products formed
    • BShown in the final equation
    • CCancelled out in final equation
    • DSpeeding up every step
    Show answer and explanation

    Answer: C) Cancelled out in final equation

    They are produced in one step and consumed in a later one. Source: Chemistry 11 KPK (taleem360.com), p. 278.

13. Question 13

    The molecularity of an elementary reaction can never be:
    • AZero
    • BOne
    • CTwo
    • DThree
    Show answer and explanation

    Answer: A) Zero

    Molecularity is a positive integer (1, 2 or 3); order can be zero. Source: Chemistry 11 KPK (taleem360.com), p. 281.

14. Question 14

    The rate constant k is equal to the rate of reaction when:
    • AThe reaction reaches equilibrium
    • BTemperature is absolute zero
    • CAll concentrations are unity
    • DNo catalyst is present
    Show answer and explanation

    Answer: C) All concentrations are unity

    Rate = k[A]^m[B]^n, so with [A] = [B] = 1, rate = k. Source: Chemistry 11 KPK (taleem360.com), p. 280.

15. Question 15

    Hydrolysis of ethyl acetate catalysed by H3O+ ions in solution is an example of:
    • AEnzyme catalysis
    • BHeterogeneous catalysis
    • CHomogeneous catalysis
    • DPhotochemical reaction
    Show answer and explanation

    Answer: C) Homogeneous catalysis

    Reactants and catalyst are in one (liquid) phase: acid-base catalysis. Source: Chemistry 11 KPK (taleem360.com), p. 294.

16. Question 16

    Hydrogenation of ethene over powdered nickel is an example of:
    • APhotochemical reaction
    • BHomogeneous catalysis
    • CEnzyme catalysis
    • DHeterogeneous catalysis
    Show answer and explanation

    Answer: D) Heterogeneous catalysis

    Solid catalyst, gaseous reactants; powdered Ni has a much larger surface area. Source: Chemistry 11 KPK (taleem360.com), p. 294.

17. Question 17

    A catalyst increases the rate of a reaction by providing a path with:
    • ALower activation energy
    • BHigher activation energy
    • CLarger enthalpy change
    • DMore reactant molecules
    Show answer and explanation

    Answer: A) Lower activation energy

    Kinetic energy unchanged, more molecules can cross the lower barrier. Source: Chemistry 11 KPK (taleem360.com), p. 293.

18. Question 18

    Ptyalin, an enzyme found in saliva, accelerates the conversion of:
    • AProtein into amino acids
    • BStarch into sugar
    • CFat into fatty acids
    • DSugar into starch
    Show answer and explanation

    Answer: B) Starch into sugar

    Without it the conversion would take weeks; pepsin breaks down proteins. Source: Chemistry 11 KPK (taleem360.com), p. 295.

19. Question 19

    In the potential energy diagram of an exothermic reaction, the reactants are:
    • ALevel with the products
    • BLower than the products
    • CHigher than the products
    • DAbove the transition state
    Show answer and explanation

    Answer: C) Higher than the products

    The difference is released as heat (negative delta H). Source: Chemistry 11 KPK (taleem360.com), p. 290.

20. Question 20

    According to collision theory, collisions with enough energy still fail to react unless the molecules have:
    • AThe same speed
    • BEqual masses
    • COpposite charges
    • DProper orientation
    Show answer and explanation

    Answer: D) Proper orientation

    Only effective collisions (energy plus orientation) lead to reaction. Source: Chemistry 11 KPK (taleem360.com), p. 291.

21. Question 21

    A reaction with a high activation energy is usually:
    • ASlow
    • BFast
    • CExplosive
    • DInstantaneous
    Show answer and explanation

    Answer: A) Slow

    Few molecules can cross a high energy barrier. Source: Chemistry 11 KPK (taleem360.com), p. 290.

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